The demands for high-performance batteries have grown significantly in various fields including mobile communications, portable electronic devices, electric automobiles, and capacitors. Lithium polymer secondary batteries, which have begun to be commercialized in recent years, include a cathode, an anode, and an electrolyte for preventing short-circuit of the two electrodes and providing ion transfer medium. The polymer electrolytes for lithium polymer secondary batteries are classified into two groups according to the type of an electrolyte: gel-type polymer electrolyte using a mixture of an organic solvent and a polymer material and solvent-free polymer electrolyte using only a polymer material.
A solvent-free polymer electrolyte has advantages of high electrochemical stability and compatibility with a high-capacity lithium metal electrode. However, due to very low ionic conductivity at room temperature, the solvent-free polymer electrolyte is still commercially unavailable. On the other hand, a gel-type polymer electrolyte contains large amounts of an electrolyte solution, and thus, has very high ionic conductivity and electrochemical characteristics comparable to a conventional liquid electrolyte. Therefore, the gel-type polymer electrolyte has been commercialized. A Bellcore process developed by Bell Communications & Research Co., which is a representative preparation process for a gel-type polymer electrolyte, is as follows.
First, polyacrylonitrile, polyethyleneoxide, or poly(vinylidene fluoride-co-hexafluoropropylene) copolymer as a matrix polymer, dibutylphthalate as a plasticizer, silicon dioxide as a filler, and an organic solvent are mixed and stirred to obtain a casting composition and the casting composition is cast to obtain a polymer film. The plasticizer is extracted from the polymer film in a subsequent process to form micro-channels in the polymer film. Then, an electrode assembly obtained by interposing the polymer film between a cathode and an anode is inserted in a can- or pouch-type case and sealed. The resultant structure is impregnated with an electrolyte solution obtained by adding a lithium salt to a mixed organic solvent in which linear carbonates and cyclic carbonates are mixed at an appropriate ratio, so that the electrolyte solution is filled in the micro-channels of the polymer film, to thereby complete battery fabrication. An electrode assembly may also be manufactured in such a way that a polymer film with micro-channels is impregnated with an electrolyte solution and then interposed between a cathode and an anode.
A lithium polymer battery employing such a gel-type polymer electrolyte contains a smaller amount of an electrolyte solution compared to a lithium ion battery employing as an electrolyte solution only a mixed organic solvent containing an appropriate ratio of linear carbonates and cyclic carbonates. However, the absolute amount of the electrolyte solution contained in the lithium polymer battery employing the gel-type polymer electrolyte is still high. Therefore, the lithium polymer battery employing the gel-type polymer electrolyte has problems such as degradation of electrochemical characteristics due to leakage or evaporation of the electrolyte solution and difficulties in safety assurance and fabrication process. In detail, the lithium polymer battery employing the gel-type polymer electrolyte may be ignited by decomposition or gasification of the electrolyte solution when heated or may undergo degradation of electrochemical properties by leakage or evaporation of the electrolyte solution.